1. Field of the Invention:
The present invention relates to a method for setting the vehicle-engine torque so as to efficiently operate the power-train components comprising an engine mounted on a vehicle in their optimum operation ranges.
2. Description of the Prior Art:
Hitherto, in a conventional motor vehicle, the engine power is transmitted to the front and/or rear wheels through the power train such as the torque converter, speed-change unit, propeller shaft, differential, and the like.
In this connection, for example, as is clear from a graph shown in FIG. 1, in locomotion of the conventional motor vehicle provided with the above power train, there is a certain relationship between traction and traveling speed of the motor vehicle. In the graph shown in FIG. 1, a torque raise is substantially within a range of from 10 to 15% in a position between a rated point "C" and a maximum-torque point "D" of the engine power output characteristics in each speed range such as 1st speed, 2nd speed, 3rd speed and the like, which torque raise is so determined as to ensure the engine durability according to requirements in engine design. On the other hand, in the speed-change unit or transmission with multiple speed ratios, each of the speed ratios between adjacent speed ranges such as those between 1st and 2nd speeds, 2nd and 3rd speeds, and like adjacent speeds of the transmission is within a range of from 1.35 to 1.40. Consequently, in order to compensate a drop in engine speed caused by such speed ratios in shifting operation of the gearshift lever of the transmission so as to keep the engine power output constant, a torque raise of from 35 to 40% is required. In a conventional vehicle engine, since the engine torque raise is within a range of from 10 to 15% as described above, the remaining part of the required torque raise is within a range of from 20 to 30% and this part is balanced or compensated for by operating the vehicle engine in its maximum engine speed regulation range. As described above, in the shifting operation, since the conventional vehicle engine is operated substantially in its maximum engine speed regulation range, the conventional vehicle engine is poor in fuel consumption for engine power output, i.e., poor in engine efficiency.
As shown in FIG. 1, the engine has its respective power output characteristics in each of the speed ranges which are shown, for example, in dotted line as to 3rd speed, in solid line as to 4th speed, and in one-dotted chain like as to 5th speed. In addition, in the case of 4th speed, the operating range of engine speed is defined between a point "P.sub.1 " at which the engine power output curve of the 4th speed crosses that of the 3rd speed and a point "P.sub.2 " at which the engine power output curve of the 4th speed crosses that of the 5th speed as shown in FIG. 1. On the other hand, as for the engine torque, the operating range of the engine torque is also defined between these points "P.sub.1 " and "P.sub.2 ".
Furthermore, it is known that the operating range in the vicinity of a point "D" at which the maximum engine power output is obtained is generally most suited for operation of the engine in improvement of the engine efficiency in any of the speed ranges.
However, as described above, in the conventional engine, the engine is operated substantially in its maximum engine speed regulation range in which the engine efficiency is poor. In addition, since frictional power loss produced in the power trains comprising a torque converter, transmission and like components varies with 1 to 2 power of engine speed, the conventional engine is disadvantageous in both of engine efficiency and power loss.